The general goal of this proposal is to develop fluorometric techniques to monitor membrane potentials of cell suspensons. These techniques may enable investigators to measure membrane potentials in cells and various membranous preparations which cannot be determined with conventional electrophysiological methods. In addition, the fluorometric technique has the advantage of yielding an average membrane potential for a population of cells whereas electrophysiological methods measure one cell at a time and require many many measurements before an average can be established. The method is based on the observation that the fluorescence intensity of a number of fluorochromes varies directly with membrane potential in thr squid axons (Cohen, 1973). Davila et al (1973) suggested that these dyes might be useful in the measurement of changes in potentials across membranes where the use of electrodes has been difficult. Laris and Hoffman (1973) followed this suggestion using a cyanine dye and human and Anphiuma erythrocytes. IN their study the fluorescence of the dye appeared to be a function of the membrane potential of these red cells. The specific aims of this proposal are: (1) to continue to test the postulate that fluorescence changes parallel, and hence, monitor membrane potential in erythrocytes. (2) to initiate studies on the question of the mechanism of the observed changes in fluorescence. (3) to determine whether or not the fluorometric methode deveoped by Laris and Hoffman (1973) are applicable to other cells. (4) to screen other fluorescent dyes for possible use in studies of membrane potential. (5) to determine whether or not the dyes have any effect on Na and K ion fluxes or on cellular integrity.